The present invention is directed toward an apparatus and method for detecting the presence of concealed eavesdropping devices. More particularly, the present invention relates to a method and apparatus for correlating a detected signal with a reference signal in order to determine the presence of a concealed eavesdropping device.
Many devices and methods are available today for detecting the presence of concealed surveillance device. One such method of attempting to detect a surveillance device is to detect the transmitted signals from an eavesdropping device present in a location and compare a demodulated transmitted signal to a reference signal, which is related to the ambient audio in the environment. If there is a strong correlation between the reference signal and the detected signal, there is a good probability that the detected signal is the result of a hidden eavesdropping device. Thus, a strong correlation between the two signals typically indicates the presence of a covert eavesdropping device that is transmitting a representation of the audio in a room to a remote location. The method of transmission from the eavesdropping device may include a wide variety of transmission medium such as electromagnetic radiation (from radio frequency broadcasts to optical transmission media such as infrared, visible, ultraviolet, LASER, etc.) to hardwired methods (such as dedicated lines, telephone lines, power lines, or any other existing wiring in an environment).
There are prior art methods for performing eavesdropping device detection. One such method of detecting an eavesdropping device is disclosed in U.S. Pat. No. 5,241,699 which is assigned to Research Electronics, Inc. and which is hereby incorporated by reference. This method relies on a phase correlator. The main drawback of a phase correlator is that it does not consider the phase distortions and time delays that result from the speed of sound and echoes from within the environment. Furthermore, it has the drawback that the phase correlator works very well for a continuous audio tone with a constant phase, but works poorly if the audio source has a very wide bandwidth with little continuous tone content.
There are also some prior art correlation methods that rely on basic mathematical cross-correlation and auto-correlation methods. In German patent application No. 24 28 299 February 1976 (Wxc3xa4chtler), there is disclosed a system for detecting eavesdropping transmitters using a cross correlation between a reference audio signal and a demodulated version of a detected transmission. In U.S. Pat. No. 5,717,656 February 1998 (Dourbal) and Russian patent No. 94025549/09 August 1995 (Dourbal), there is disclosed a method for detecting eavesdropping transmitters using cross correlation and autocorrelation functions. These patents by Dourbal (US and Russian) also include a method for detecting the range to the eavesdropping device. However, these prior art correlation methods suffer from a number of drawbacks. For example, the prior art devices utilize basic cross-correlation to determine the correlation between the reference signal and the detected signal. However, basic cross-correlation only provides a resulting correlation number. This number simply indicates that the signals are, or are not, correlated. Thus, basic cross-correlation and auto-correlation techniques do not provide robust resultant information about the relationship between the signals because these prior art correlation techniques do not work well in large rooms with large time delays and phase distortions resulting from echoes. Furthermore, the prior art correlation techniques that incorporate a ranging function or locating function have some drawbacks. First of all, the prior art basically relies on measuring the timed response of the initiation of a known sound source and comparing the time of arrival difference between the reference signal and the intercept signal. This method of range determination also does not work well in large rooms with large time delays and phase distortions resulting from echoes. Therefore, the cross-correlation of the prior art is deficient in a number of respects. Furthermore, all known prior art correlation methods fail to provide the ability to evaluate the frequency response of the intercepted signal and/or the reference audio signal and to automatically introduce a filtering capability into the correlation process to improve upon the correlation data and range finding capabilities of the process.
Yet another problem with prior art devices is that the correlation methods utilized are highly dependent upon unpredictable signal and noise amplitude levels in the room. Thus, it may be necessary to introduce particular types of reference audio into the room in order for the prior art correlation methods to provide any type of useful results. However, producing unusual noises in the room may tip off a third party that is monitoring the room that a detection attempt is being made. This information may result in the third party turning off the surveillance device and frustrating any attempts to locate the device. Thus, the prior art correlation methods create a risk of detection by alerting a third party operating the surveillance device.
Therefore, in view of the above discussed deficiencies in the prior art, what is needed is an advanced correlation method and apparatus which require few mathematical calculations to implement. The invention should not require the introduction of an easily detectable noise source into the room to be scanned. In addition, the correlation method and apparatus should allow the user to determine the surveillance device""s location.
The present invention overcomes the above discussed deficiencies of the prior art by providing a new correlation method which is implemented using discrete Fourier transforms. One skilled in the art will appreciate that discrete Fourier transforms are utilized when dealing with sampled data. While there are many discrete Fourier transform algorithms, the preferred embodiment of the invention utilizes a Complex Fast Fourier Transform to minimize the required calculations. In addition, the data sets calculated are preferably normalized after each Fourier transform process to ensure that the amplitude levels of the data sets do not affect the resulting correlation data. This process is referred to as Fast Correlation.
In particular, the preferred embodiment of the present invention is directed toward a method for detecting the presence of an electronic eavesdropping device that is generating a transmission signal corresponding to sounds in an environment. In accordance with the method, ambient sounds in the environment are detected. Alternatively, a sound source may be generated to produce ambient sounds that would be non-alerting to any third party that may be eavesdropping on the environment. A reference signal corresponding to the ambient sounds in the environment is then generated. An intercept signal corresponding to a detected transmission signal present in the environment is also generated. The reference signal and the intercept signal are sampled to produce sampled reference signal data and sampled intercept signal data. The reference signal is then compared to the intercept signal to determine if an electronic eavesdropping device is present in the environment by performing a fast correlation process on the sampled data.
The preferred embodiment of the invention utilizes a fast correlation process that includes the taking of a Fourier Transform of the sampled reference signal data and an Inverse Fourier Transform of the sampled intercept signal data. The Inverse Fourier Transform of the sampled intercept signal data and the Fourier Transform of the sampled reference data are multiplied to produce product data. The resulting product data is a frequency representation of the correlation of the reference signal and the intercept signal. This product is then filtered by multiplying the product by a weighting function. The simplest weighting function would be one that simply zeroes the frequencies of no interest. The Inverse Fourier Transform of the product data is taken to produce the fast correlation data. The resulting fast correlation data is graphed to provide a graphic display of the correlation between the original two sets of data for all possible time shifts within the sampled time window.
It is important to note that while the preferred embodiment utilizes an Inverse Fourier transform of the sampled intercept data, there are other mathematical approaches to address the correlation process described in the present invention. For example, the Inverse Fourier transform of the sampled reference data could be taken instead of the Inverse Fourier transform of the sampled intercept data. In this case, the resulting correlation data will be reversed. Alternatively, either data set could simply be reversed and basic Fourier transforms could be implemented for the initial Fourier transform process. Yet another approach is to utilize a known audio sound source and have a reference data set that is stored in memory for use in the correlation process. This process is referred to as fast convolution. The major difference between fast correlation and fast convolution is the data reversal of one of the initial sets being evaluated. The correlation process has a data reversal which is implemented in this invention using an Inverse Fourier transform to minimize steps while the convolution process has no data reversal.
The above discussed embodiment improves upon the prior art in a number of respects. For example, the method provides correlation data that has a large number of data points that provide additional information concerning the compared signals as opposed to a single correlation number. In particular, this correlation data gives the correlation between the two sets of sampled data for all possible time shifts. This approach therefore addresses and takes advantage of the echoes and phase shifts that uniquely occur in various environments due to the size, shape, and interior contents of the environment of concern. Furthermore, the above method results in a set of correlation data that assumes a periodic nature of the sampled signals. This is advantageous in that the resultant correlation data provides information corresponding to time shifts in both the negative and positive directions between the two sets of data while keeping the amplitude level independent of the number of points that are multiplied. The fast correlation method provides this improved correlation data for all possible time shifts while requiring many less calculations than the prior art methods.
In other alternative embodiments of the present invention, zero padding techniques are used in the fast correlation process to increase resolution of the resultant correlation data. This approach is implemented by increasing the sample size of the reference and intercept signals by simply adding zero value samples to the data sets. These zero value samples can be added to the original sampled data sets prior to any processing to increase the frequency spectrum resolution. Or, zero value samples can be added to the filtered data set prior to taking the final Inverse Fourier transform to increase the range resolution of the final fast correlation data.
The sampled reference signal data and sampled intercept signal data are preferably filtered such that frequencies in a frequency band of interest will be emphasized in the fast correlation process. The filter function parameters for the filtering are automatically determined by analyzing frequency spectrum data from the reference signal and the intercept signal. The filter function can be automatically determined by developing an algorithm that evaluates the Fourier transform of the intercept signal, and then de-emphasizes the portions of the frequency spectrum where the detected eavesdropping device appears to have poor frequency response. This approach further optimizes the correlation process, and is a large improvement over the prior art correlation methods.
The present invention also comprehends a variety of different ways of generating the reference signal. In one such embodiment, the ambient sounds in the environment are measured with a microphone to produce the reference signal. In another embodiment, a telephone signal is received from a telephone line that corresponds to the ambient sounds in the environment sensed by the telephone and the telephone signal is used as the reference signal. In yet another embodiment, an audio signal produced by an audio source is introduced into the environment and an output signal corresponding to the audio signal from the audio source is used as the reference signal. For reasons discussed in more detail below, the audio signal introduced by the audio source into the environment is preferably audio white noise.
The above discussed system improves upon the prior art by providing the ability to correlate on ambient room noises such as air conditioner noise or street noise coming in through a window without the introduction of an audio signal. In addition, the use of an audio source for the reference signal provides an advantage by generating a reference signal that does not contain any echoes or phase distortions resulting from the environment. White noise is preferably utilized because it provides a very broad audio frequency spectrum which compliments the fast correlation process while providing an ambient environment audio source that is non-alerting to a third party because it contains no intelligible audio content. For example, a specialized noise such as a series of beeps might alert a third party listening through the concealed surveillance device that a counter intelligence operation is being undertaken. This knowledge may lead the party operating the surveillance device to turn off the device, thereby frustrating attempts to locate the device. Thus, the ability to locate a surveillance device with a non-alerting audio source such as white noise is a substantial improvement upon the prior art.
Embodiments such as those discussed above which introduce audio signals into the environment of interest allow the fast correlation data to be analyzed to determine a range to a microphone of the electronic surveillance device which can be displayed to a user. The correlation data is graphed in such a manner that a first peak in the graph represents the distance from the audio source to the electronic surveillance device. Furthermore, multiple range measurements may be taken and the location of the electronic surveillance device determined through triangulation based upon the multiple range measurements.
There are also a variety of methods of generating an intercept signal in accordance with the present invention. In one embodiment, electromagnetic signals in the environment are monitored with an electronic receiver that generates an intercept signal that corresponds to the electromagnetic signals in the environment. In another embodiment, electronic signals are received from any miscellaneous lines in the environment such as power lines, telephone lines, LAN lines, or security system wires, and an intercept signal is produced that corresponds to the electronic signals on the lines in the environment. In yet another embodiment, electronic signals are received from a light wave detector. This light wave detector may be capable of infrared, visible, ultraviolet or laser frequencies. Regardless of the method used to intercept the transmission signals, the preferred embodiment of the present invention uses the fast correlation data derived from the intercept and reference signals to calculate a correlation figure of merit such that the value of the correlation figure of merit indicates the presence of an electronic surveillance device. In this preferred embodiment, the figure of merit is calculated by evaluating the statistical characteristics of the resulting fast correlation data. Furthermore, in especially preferred embodiments, the figure of merit is calculated using standard deviation calculations such that the figure of merit is normalized and does not depend upon the signal amplitude levels of the suspected surveillance device signal and the reference signal. In addition, the fast correlation data and the calculated figure of merit data may be integrated to smooth out the resulting data. This is helpful in minimizing any drastic changes that may occur in the results of the correlation process due to rapid changes in the audio spectrum of the environment.
The present invention is also directed toward an apparatus for detecting the presence of a surveillance device in an environment of interest. The apparatus includes a reception device for intercepting transmission signals being generating by an eavesdropping device and producing an intercept signal that corresponds to the demodulated audio from the transmission signal. The apparatus also produces an electric reference signal that corresponds to the sounds propagating through the environment. Samplers simultaneously sample the electric intercept signal and the electric reference signal during a sampling interval to produce digital intercept data and digital reference data for processing.
The preferred apparatus also has a processor. The processor performs an Inverse Fast Fourier Transform on the digital intercept data and a Fast Fourier Transform on the digital reference data to produce transformed intercept data and transformed reference data. The processor may also take the Inverse Fast Fourier Transform of the digital intercept data by reversing the order of data bits in the digital intercept data and taking a Fast Fourier Transform of the reversed order digital intercept data. The product of the transformed reference data and the transformed intercept data is calculated to produce product data. This data can be filtered in order to further improve the correlation process by emphasizing the audio frequencies of interest. This filtering can be implemented separately or incorporated into the functioning of the processor. The Inverse Fast Fourier Transform of the product data is then taken to produce the fast correlation data. The processor produces a graph of the fast correlation data and a display displays the graph to a user. The fast correlation data is then examined to determine if the demodulated audio from the intercepted signal corresponds to the sound propagating through the environment. The preferred apparatus further includes a range determining algorithm that determines a range to the surveillance device based upon the fast correlation data. Alternatively, the range determining algorithm may determine the range to the surveillance device based upon the graph of the fast correlation data. A filter, having filtering parameters selected based on utilizing a specific portion of the audio frequency spectrum for correlation, frequency filters the fast correlation data. Alternatively, the filtering parameters may be automatically determined by the processor based upon a power spectral density of a frequency spectrum of the intercept signal. The processor may perform triangulation calculations to determine the location of the surveillance device through triangulation based upon multiple range determinations.
One advantage of the above discussed method over the prior art is that it has very robust performance in different acoustic environments. The performances of the prior art correlation methods and apparatus are highly dependent upon unpredictable signal levels. However, the preferred embodiment of the present invention is not dependent upon unpredictable signal amplitude levels because all of the signal levels are preferably normalized through out the correlation process and in the figure of merit calculations. Furthermore, the sensitivity of the correlation process described in the present invention depends highly on the amount of time that is sampled to generate the original intercept signal and reference signal. Therefore, the sensitivity of the correlation process can be greatly improved by simply sampling the intercept signal and the reference signal for a longer period of time before implementing the fast correlation process. And, the sensitivity of the correlation process can be further improved by implementing the digital audio filtering process described herein.
As can be seen from the previous discussion, the present invention provides an array of improvements over the prior art. Yet other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.